The present invention relates to electroplating, and more particularly to an improved anode for analyzing organic additives in an electroplating bath.
Electroplating is a complex process involving an electroplating bath with multiple constituents. Acid copper electroplating baths, for example, contain constituents such as a source of copper, a source of acid, optionally a low level of chloride, and organic additives such as accelerators, suppressors, and levelers. It is important that the concentration of these constituents be kept within close tolerances to obtain a high-quality deposit. In some cases, the levels of additives are adjusted based on “rules of thumb” developed over time. In other cases, analysis of individual constituents can be made regularly and additions made as required, for example pH measurements made for acid content. However, organic constituents such as brighteners, leveling agents, suppressors, and the like, together with various impurities that can affect the quality of a deposit, are more difficult to individually analyze on an economic and/or timely basis, especially when the operating concentrations of these materials are low (e.g., less than 1 part per million by weight).
U.S. Pat. No. 4,134,605 to Tench is directed to a method of evaluating concentrations of components typically found in electroplating baths. The method is based on an electrochemical cell with a working electrode that functions as the cathode during copper deposition, a counter electrode that functions as the anode during deposition, and a reference electrode that is immersed in the analyte solution from an electroplating bath. Typically, the working electrode is inert in the bath, and may be a rotating disk to maintain relative motion between the bath and the electrode itself in order to maintain a constant flux of bath components at the working electrode. A voltammetric cycle “sweep” potential is applied between the working electrode and the counter electrode, wherein the sweep is controlled by a function generator. The counter electrode is coupled in series with the function generator to form a coulometer used to measure the current utilized during various portions of the voltammetric cycle.
A calibration curve is produced by sweeping the working electrode through the voltammetric cycle in a series of electroplating baths of known analyte concentration. A portion of the current-use profile generated by each of these bath analyses is then correlated to the concentration of the analyte of interest. Analysis of a bath with an unknown concentration of this analyte can then be determined by comparing a measured, current-use profile to that of the calibration curve. Accordingly, the success of determining an analyte concentration is directly tied to the precision with which the analysis can be produced.
However, during continuous use of an electroplating bath, and following successive analyses, contaminants often accumulate on the electrodes, which has a detrimental effect on the analysis. Attempts to decrease or prevent contamination on the working electrode include sequentially pulsing the working electrode between appropriate metal plating, metal stripping, and cleaning operations, and by applying an equilibrium potential between pulses to maintain a clean and reproducible surface. In particular, U.S. Pat. No. 4,917,774 to Fisher is directed to preventing buildup on the working electrode by using a pulsed sweep without applying a potential following each completed cycle, or by applying a potential equal to or approximately equal to the open circuit potential of the working electrode in the bath following the cycle of metal plating, metal stripping, and cleaning.
However, such methods are only a partial solution, as they do not address accumulation of organic by-products and other bath materials on the counter electrode. This phenomenon manifests itself in both inaccurate (10–30% error) and irreproducible analyses for various analytes. This is especially true when the component concentration must be determined in a solution containing a relatively large excesses of other bath components (e.g., determining the concentration of a leveler in the presence of accelerators and suppressors). Accordingly, a counter electrode capable of sustained use in an electroplating bath would be beneficial, and in particular, a counter electrode that resists becoming coated or otherwise obstructed by organic materials present in the electroplating bath.